Dendritic cells (DC) perform an essential role in the generation and regulation of antitumor immune responses. It is therefore not surprising that many tumors have evolved mechanisms that alter DC activity and function. Our recent work indicates that the generation, maturation, function, and longevity of DC are all markedly inhibited in cancer. However, molecular mechanisms and intracellular pathways that are involved in the inhibition of DC differentiation and maturation in cancer have not yet been explored. Awareness of these mechanisms will help to direct development of the next generation of tumor vaccines. Novel therapies that can correct DC dysfunction without compromising normal host cell-mediated immunity are desirable. Hence, the identification of potential molecular targets in DC and/or DC precursors, amenable to therapeutic manipulation, is critical. Our main goal is to determine how tumor escapes immune recognition in order to uncover useful molecular targets for the therapeutic protection of the immune system. The purpose of this application is to characterize novel molecular mechanisms that inhibit DC maturation in cancer. Our major hypothesis is that the small Rho GTPases from the Ras superfamily of GTP-binding proteins are important tumor targets that are responsible for regulating DC maturation and function in the tumor microenvironment. On the basis of our preliminary data, we also hypothesize that the dysbalance of DC maturation induced by tumors is associated with impaired endocytosis, migration, adhesion, chemotaxis, and antigen processing and presentation by DC and that Rho GTPases are directly involved in these phenomena. We propose to evaluate tumor-mediated regulation of Rho, Rac, and Cdc42 in DC and their role in regulating DC maturation and function in cancer. To test our hypotheses, we will pursue four Specific Aims: 1. Identify pathways involved in inhibiting DC maturation and function in the tumor microenvironment by identifying GTP-GDP cycling-dependent and -independent mechanisms that regulate Rho GTPases in DC. 2. Assess the effect that specific signaling pathways have on deficient DC maturation in cancer by evaluating the role of small Rho GTPases in regulating DC function. 3. Identify signaling to Rho GTPases in DC in cancer by comparing and contrasting the effects of purified DC suppressing tumor-derived factors and DC activating cytokines. 4. Evaluate the therapeutic feasibility of targeting the Rho GTPase network in DC using animal tumor models. By accomplishing these aims, we will have proposed a new molecular mechanism of the regulation of dendropoiesis in cancer. Additionally, understanding the mechanisms responsible for DC dysfunction in cancer will serve as a basis for developing novel therapeutic approaches for patients with cancer.